Extension above garage is cold. More insulation?

22 Mar 2006
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United Kingdom

A previous owner extended our house and put an extra bedroom over the garage (around 15 years ago).

The problem I have is that this bedroom is the coldest room in the house.

When we moved in we put a new central heating system in including radiators so the rad should be the right size for the bedroom.

It seems that the cold is coming from the floor. Below is the garage and is always cold as you would expect.

When the radiator was put into the bedroom I did notice that there was some rockwool insulation under the floor boards, so it does have some insulation but I guess its not up to the job.

Ideally I would like to take up the floor and redo the insulation in the bedroom floor, but that involves quite a bit of hassle. All the furniture in the room will need dismantling. Also the floor is made of chipboard so it's likely to break whilst pulling it up and need replacing.

So instead of insulating the bedroom floor, can I insulate the Garage ceiling with Insulation Board??

What thickness of board would be required?


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You can insulate from below.

A typical garage ceiling would have say 12.5mm fireline plasterboard, 50mm celotex insulation across the joists and 150mm glass wool between.
So it would look like this:

_____________________ BEDROOM Floor
|xxxxxxx|xxxxxxx|xxxxxxx| Joist with 150mm wool insulation
================== 50mm Insulation board
--------------------------------- 12.5mm Plasterboard

So you would nail/screw the plasterboard through the insulation board into the joist?

TBH, I was thinking of just sticking some insulation board to the existing ceiling of the garage then platerboard over that. Obviously this will reduce the height of the garage.
You have got the gist of it yes.

We would normally tack the insulation in place with about six fixings then fix the plasterboard through the inso' with say 90mm drywall screws.

You may find it difficult to find screws long enough to do it using your method.
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You have got the gist of it yes.

We would normally tack the insulation in place with about six fixings then fix the plasterboard through the inso' with say 90mm drywall screws.

You may find it difficult to find screws long enough to do it using your method.
I have seen some 100mm screws that I might get away with.

Or... I could add some 50mm joists to the ceiling fill in between with insulation board and the plasterboard over that. Quite a bit more work with that method.

Thanks for your help.
Why would you want to buy a product from a company that doesn;t know what it is talking about - or lying ?

That table in your link is a mess with lots of misleading, wrongly-labelled data given in differing units.

Also no attribution to the body that has carried out the testing which is very unusual.

Are you connected to them . Texx ?
No need to be so aggressive it was just a suggestion, I was going to use it in my house that I am renovating and thought it sounded ideal fo this kind of situation. I am not linked in any way to this company, and why is the info lies, need to know for my own sake
The Superquilt looks easier to put up and the cost is around the same as Celotex or Kingspan boards.

What I can't work out is which provides the best insulation!
As I can't find any data where the two are compared or use the same values.

It doesn't look like a dodgy product though. The ecohome-insulation website is a bit naff. The manufacturer websiste is better.

The thing with insulation, is that it should be used to keep heat in and not to keep cold out.

So it should be placed as close a possible to the heated room

By insulating the garage ceiling, there is still the issue of the cold floor void and possible air leakage between the joists

The best solution would be to remove the ceiling, and fit the celotex tight up the the floor deck, and then fill the remaining void with quilt insulation

Same principle if considering superquilt or tri-iso - even ignoring theoretical performance in comparison with traditional insulation, you will be insulating the garage and not the room above ... so it would be the wrong type of insulation in this use because it is not designed for this type of installation
@ texx

You're right , it does come across agressively, apologies.

The reason why is because thin-foil insulation is, in my opinion, one of the two areas in DIY that there is an awful lot of conning going on ( damp-proof the other)

It is also often the case that when someone joins the forum and posts on the same day and that post recommends a product, they are often associated with that company.

As I said the technical information is a mess, sometimes designated in the wrong units, sometimes attributing wrong values to products ( like the Celotex). Generally intended to bamboozle rather than inform.

I see from the pricing that they are now selling it at a higher price than Celotex even though in their own matrix they show the Celotex as being more effective.

http://www.ecohome-insulation.com/files/ww/SuperQuilt Roof Data Sheet (Eco-Home).pdf

Look at the line PIR with a value of 2.995 and then the three lines below giving a value of 2,71 for the YBS - approx 10% worse for same thickness.

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